Process for producing highly conducting and transparent films from graphene oxide-metal nanowire hybrid materials

ABSTRACT

A process for producing a transparent conductive film, comprising (a) providing a graphene oxide gel; (b) dispersing metal nanowires in the graphene oxide gel to form a suspension; (c) dispensing and depositing the suspension onto a substrate; and (d) removing the liquid medium to form the film. The film is composed of metal nanowires and graphene oxide with a metal nanowire-to-graphene oxide weight ratio from 1/99 to 99/1, wherein the metal nanowires contain no surface-borne metal oxide or metal compound and the film exhibits an optical transparence no less than 80% and sheet resistance no higher than 300 ohm/square. This film can be used as a transparent conductive electrode in an electro-optic device, such as a photovoltaic or solar cell, light-emitting diode, photo-detector, touch screen, electro-wetting display, liquid crystal display, plasma display, LED display, a TV screen, a computer screen, or a mobile phone screen.

FIELD OF THE INVENTION

The present invention relates generally to the field of transparentconductive electrodes for solar cell, photo-detector, light-emittingdiode, touch screen, and display device applications and, moreparticularly, to a graphene/metal nanowire-based hybrid film with acombination of exceptional optical transparency and high electricalconductivity (or low sheet resistance).

BACKGROUND OF THE INVENTION

The following references are related to the art of “transparent andconductive electrodes”:

-   1. L. Hu, D. S. Hecht, and G. Gruner, “Percolation in Transparent    and Conducting Carbon Nanotube Networks,” Nano Letters, 2004, 4,    2513-2517.-   2. Z. Wu, et al. “Transparent, Conductive Carbon Nanotube Films,”    Science 27 Aug. 2004: Vol. 305 no. 5688 pp. 1273-1276.-   3. H. G. Park, et al., “Transparent Conductive Single Wall Carbon    Nanotube Network Films for Liquid Crystal Displays, ECS Solid State    Lett. 2 Oct. 2012: R31-R33.-   4. Jung-Yong Lee, Stephen T. Connor, Yi Cui, and Peter Peumans,    “Solution-Processed Metal Nanowire Mesh Transparent Electrodes,”    Nano Letters, 2008, 8 (2), pp 689-692.-   5. S. De, et al., “Silver Nanowire Networks as Flexible,    Transparent, Conducting Films: Extremely High DC to Optical    Conductivity Ratios,” ACS Nano, 2009, 3, 1767-1774.-   6. Ting-Gang Chen, et al., “Flexible Silver Nanowire Meshes for    High-Efficiency Microtextured Organic-Silicon Hybrid Photovoltaics,”    ACS Applied Materials & Interfaces, 2012, 4 (12), 6857-6864.-   7. Taegeon Kim, et al., “Electrostatic Spray Deposition of Highly    Transparent Silver Nanowire Electrode on Flexible Substrate, ACS    Appl. Mater. Interfaces, Article ASAP; DOI: 10.1021/am3023543.-   8. Y. Ahn, Y. Jeong, and Y. Lee, “Improved Thermal Oxidation    Stability of Solution-Processable Silver Nanowire Transparent    Electrode by Reduced Graphene Oxide,” ACS Applied Materials &    Interfaces, 2012, 4 (12), 6410-6414.-   9. G. Gruner, L. Hu, and D. Hecht, “Graphene Film as Transparent and    Electrically Conductive Material,” US Patent Pub. No. 2007/0284557    (Dec. 13, 2007).-   10. L. Hu, et al., “Touch Screen Devices Employing Nanostructure    Network,” US Patent Pub. No. 2008/0048996 (Feb. 28, 2008).-   11. G. Gruner, et al.; “Graphene Film as Transparent and    Electrically Conductive Material,” US Patent Pub. No. 2009/0017211    (Jan. 15, 2009).-   12. G. Eda, et al., “Large-Area Ultrathin Films of Reduced Graphene    Oxide as a Transparent and Flexible Electronic Material. Nature    Nanotechnology, 2008, 3, 270-274.-   13. X. Wang, L. Zhi, and K. Mullen, “Transparent, Conductive    Graphene Electrodes for Dye-Sensitized Solar Cells. Nano Letters,    2008, 8, 323.-   14. J. B. Wu, et al., “Organic Light-Emitting Diodes on    Solution-Processed Graphene Transparent Electrodes,” ACS Nano 2009,    4, 43-48.-   15. S. De and J. N. Coleman, “Are There Fundamental Limitations on    the Sheet Resistance and Transparence of Thin Graphene Films?” ACS    Nano, 2010 May 25; 4(5), pp. 2713-20.-   16. K. S. Kim, et al. “Large-Scale Pattern Growth of Graphene Films    for Stretchable Transparent Electrodes,” Nature, 2009, 457, 706-710.-   17. X. S. Li, et al. “Transfer of Large-Area Graphene Films for    High-Performance Transparent Conductive Electrodes,” Nano Letters,    2009, 9, 4359-4363.-   18. A. Reina, et al. “Large Area, Few-Layer Graphene Films on    Arbitrary Substrates by Chemical Vapor Deposition,” Nano Letters,    2009, 9, 30-35.-   19. Sukang Bae, et al., “Roll-to-roll production of 30-inch graphene    films for transparent electrodes,” Nature Nanotechnology, Vol. 5,    August 2010, 574-578.-   20. V. C. Tung, et al. “Low-Temperature Solution Processing of    Graphene-Carbon Nanotube Hybrid Materials for High-Performance    Transparent Conductors” Nano Letters, 2009, 9, 1949-1955.-   21. I. N. Kholmanov, et al. “Improved Electrical Conductivity of    Graphene Films Integrated with Metal Nanowires,” Nano Letters, 2012,    12 (11), pp 5679-5683.

Optically transparent and electrically conductive electrodes are widelyimplemented in optoelectronic devices, such as photovoltaic (PV) orsolar cells, light-emitting diodes, organic photo-detectors, and variousdisplay devices. For use in these applications, the electrode materialsmust exhibit both exceptionally high optical transmittance and low sheetresistance (or high electrical conductivity). More commonly usedtransparent and conductive oxides (TCO) for the electrodes in thesedevices include (a) indium tin oxide (ITO), which is used for organicsolar cells and light-emitting diodes, and (b) Al-doped ZnO, which isused in amorphous solar cells. There are some alternatives to these TCOthat are being considered, such as single-walled carbon nanotubes (CNT),graphene, and metal or metal nanowires (NW).

Discrete carbon nanotubes may be used to form a thin film of highlyporous network (or mesh) of electron-conducting paths on an opticallytransparent substrate, such as glass or polymer (e.g., polyethyleneterephthalate, PET or polycarbonate). The empty spaces between nanotubesallow for light transmission and the physical contacts between nanotubesform the required conducting paths [Refs. 1-3]. However, there areseveral major issues associated with the use of CNTs for making atransparent conductive electrode (TCE). For instance, a higher CNTcontent leads to a higher conductivity, but lower transmittance due to alower amount of empty spaces. Further, the sheet resistances ofCNT-based electrodes are dominated by the large CNT junction resistancesdue to the mixed carbon nanotube varieties, with ⅓ being metallic and ⅔semiconducting. As a result, a typical sheet resistance of CNT networkson a plastic substrate is 200-1,000 ohms/square (Ω/□) at an opticaltransmittance of 80-90%. The relatively high sheet resistance, comparedwith the approximately 10-50 ohms/square of high-end ITO on a plasticsubstrate, is far from being adequate for the practical application oftransparent CNT electrodes in current-based devices, such as organiclight emitting diodes and solar cells. Furthermore, an opticaltransmittance of >90% is generally required for these devices. Even forvoltage-driven devices, such as capacitive touch screens,electro-wetting displays, and liquid crystal displays, a relatively lowsheet resistance is highly desirable.

Metal nanowire mesh-based conductive and transparent films are alsobeing considered as a potential replacement for ITO [Refs. 4-8].However, metal nanowires also suffer from the same problems as CNTs. Forinstance, although individual metal nanowires (e.g. Ag nanowires) canhave a high electrical conductivity, the contact resistance betweenmetal nanowires can be significant. Additionally, although Ag nanowirefilms can show good optical and electrical performance, it has beendifficult to make Ag nanowires into a free-standing thin film or a thinfilm of structural integrity coated on a substrate. In particular, Agnanowire films that are deposited on a plastic substrate exhibitunsatisfactory flexibility and mechanical stability in that thenanowires can easily come off. Also, the surface smoothness is poor(surface roughness being too large).

Furthermore, all metal nanowires still have a long-term stability issue,making them unacceptable for practical use. When Ag nanowire films areexposed to air and water, Ag nanowires can be easily oxidized, leadingto sharp increase in sheet resistance and hazeness of the films. Ahn, etal [Ref. 8] disclosed the deposition of a reduced graphene oxide (RGO)layer or multiple RGO layers to a pre-fabricated Ag nanowire layer. Theintent was to protect the underlying Ag nanowire film, but this approachcan introduce additional issues to the film, e.g. significantly reducedoptical transmittance by carrying out multiple coating passes andincreased sheet resistance (when the Ag nanowire film was coated withmore than 3 passes).

Graphene is yet another potential alternative to ITO. An isolated planeof carbon atoms organized in a hexagonal lattice is commonly referred toas a single-layer graphene sheet. Few-layer graphene refers to a stackof up to 5-10 planes of hexagonal carbon atoms bonded along thethickness direction with van der Waals forces. The generally goodoptical transparency and good electrical conductivity of graphene havemotivated researchers to investigate graphene films for transparent andconductive electrode (TCE) applications [Refs. 9-21].

For instance, Gruner et al [Refs. 9-11] suggested a transparent andconductive film comprising at least one network of “graphene flakes,”which are actually very thick graphite flakes. A suspension of graphiteflakes in a solvent was deposited onto a transparent glass, allowingisolated graphite flakes to somehow overlap one another to form a mesh(e.g. FIG. 1 of Ref. 9 and FIG. 1 of Ref. 11). The empty spaces betweengraphite flakes permit the light to pass through. However, these filmstypically exhibit a sheet resistance as high as 50 kOhm/square(5×10⁴Ω/□) at 50% transparency. The low transparency is a result ofusing thick graphite flakes, not graphene sheets. Gruner et al thenattempted to improve the film performance by combining carbon nanotubesand graphite flakes to form an interpenetrating network of conductivepathways (e.g. FIG. 2 of Ref. 9 and FIG. 2 of Ref. 11). Unfortunately,the interpenetrating network of graphite flakes and carbon nanotubeslead to a film that is only 80% transparent at 2 kOhms/square or 65%transparent at 1 kOhms/square (e.g., paragraph [0026] in both Ref. 9 andRef. 11). These values are absolutely unacceptable to the TCE industry.

In a graphene film made by metal-catalyzed chemical vapor deposition(CVD), each graphene plane loses 2.3-2.7% of the optical transmittanceand, hence, a five-layer graphene sheet or a film with five single-layergraphene sheets stacked together along the thickness direction wouldlikely have optical transmittance lower than 90%. Unfortunately,single-layer or few layer graphene films, albeit optically transparent,have a relatively high sheet resistance, typically 3×10²-10⁵ Ohms/square(or 0.3-100 kΩ/□). The sheet resistance is decreased when the number ofgraphene planes in a film increases. In other words, there is aninherent tradeoff between optical transparency and sheet resistance ofgraphene films: thicker films decrease not only the film sheetresistance but also the optical transparency.

A recent study [Ref. 19] has demonstrated that single-layer CVD graphenefilms can have sheet resistances as low as ˜125Ω/□ with 97.4% opticaltransmittance. However, the sheet resistance is still lower thandesirable for certain applications. The authors further usedlayer-by-layer stacking to fabricate a doped four-layer film that showssheet resistance at values as low as ˜30Ω/□ at ˜90% transparency, whichis comparable to those of certain ITO grades. However, thelayer-by-layer procedure is not amenable to mass production oftransparent conductive electrodes for practical uses. Doping also addsan extra level of complexity to an already highly complex andchallenging process that requires a tight vacuum or atmosphere control.The CVD process and equipment are notoriously expensive. Strong andurgent needs exist for more reliable and lower-cost processes and/or TCEmaterials that exhibit outstanding performance (e.g. sheet resistance<30-40Ω/□, yet still maintaining a transparency no less than 90%).

Since both graphene and carbon nanotube (CNT) have carbon atoms as theprimary element, it is appropriate to briefly discuss carbon-basedmaterials at this juncture. Carbon is known to have five uniquecrystalline structures, including diamond, fullerene (0-D nano graphiticmaterial), carbon nano-tube or carbon nano-fiber (1-D nano graphiticmaterial), graphene (2-D nano graphitic material), and graphite (3-Dgraphitic material). The carbon nano-tube (CNT) refers to a tubularstructure grown with a single wall or multi-wall. Carbon nano-tubes(CNTs) and carbon nano-fibers (CNFs) have a diameter on the order of afew nanometers to a few hundred nanometers. Their longitudinal, hollowstructures impart unique mechanical, electrical and chemical propertiesto the material. The CNT or CNF is a one-dimensional nano carbon or 1-Dnano graphite material.

Bulk natural flake graphite is a 3-D graphitic material with eachparticle being composed of multiple grains (a grain being a graphitesingle crystal or crystallite) with grain boundaries (amorphous ordefect zones) demarcating neighboring graphite single crystals. Eachgrain is composed of multiple graphene planes that are oriented parallelto one another. A graphene plane in a graphite crystallite is composedof carbon atoms occupying a two-dimensional, hexagonal lattice. In agiven grain or single crystal, the graphene planes are stacked andbonded via van der Waal forces in the crystallographic c-direction(perpendicular to the graphene plane or basal plane). Although all thegraphene planes in one grain are parallel to one another, typically thegraphene planes in one grain and the graphene planes in an adjacentgrain are different in orientation. In other words, the orientations ofthe various grains in a graphite particle typically differ from onegrain to another.

The constituent graphene planes of a graphite crystallite can beexfoliated and extracted (or isolated) to obtain individual graphenesheets of carbon atoms provided the inter-planar van der Waals forcescan be overcome. An isolated, individual graphene sheet of carbon atomsis commonly referred to as single-layer graphene. A stack of multiplegraphene planes bonded through van der Waals forces in the thicknessdirection with an inter-graphene plane spacing of 0.3354 nm is commonlyreferred to as a multi-layer graphene. A multi-layer graphene platelethas up to 300 layers of graphene planes (<100 nm in thickness). When theplatelet has up to 5-10 graphene planes, it is commonly referred to as“few-layer graphene” in the scientific community. Single-layer grapheneand multi-layer graphene sheets are collectively called “nano grapheneplatelets” (NGPs). Graphene sheets/platelets (NGPs) are a new class ofcarbon nano material (a 2-D nano carbon) that is distinct from the 0-Dfullerene, the 1-D CNT, and the 3-D graphite.

Our research group pioneered the development of graphene materials andrelated production processes as early as 2002: (1) B. Z. Jang and W. C.Huang, “Nano-scaled Graphene Plates,” U.S. Pat. No. 7,071,258 (Jul. 4,2006), application submitted on Oct. 21, 2012; (2) B. Z. Jang, et al.“Process for Producing Nano-scaled Graphene Plates,” U.S. patentapplication Ser. No. 10/858,814 (Jun. 3, 2004); and (3) B. Z. Jang, A.Zhamu, and J. Guo, “Process for Producing Nano-scaled Platelets andNanocomposites,” U.S. patent application Ser. No. 11/509,424 (Aug. 25,2006).

It may be noted that NGPs include discrete sheets/platelets ofsingle-layer and multi-layer pristine graphene, graphene oxide, orreduced graphene oxide with different oxygen contents. Pristine graphenehas essentially 0% oxygen. Graphene oxide (GO) has 0.01%-46% by weightof oxygen and reduced graphene oxide (RGO) has 0.01%-2.0% by weight ofoxygen. In other words, RGO is a type of GO having lower but non-zerooxygen content. Additionally, both GO and RGO contain a high populationof edge- and surface-borne chemical groups, vacancies, oxidative traps,and other types of defects, and both GO and RGO contain oxygen and othernon-carbon elements, e.g. hydrogen [Ref. 14; J. B. Wu, et al]. Incontrast, the pristine graphene sheets are practically defect-free andcontain no oxygen. Hence, GO and RGO are commonly considered in thescientific community as a class of 2-D nano material that isfundamentally different and distinct from pristine graphene.

It may be further noted that CVD graphene films, although relativelyoxygen-free, tend to contain a significant amount of other non-carbonelements, such as hydrogen and nitrogen. The CVD graphene ispolycrystalline and contains many defects, e.g., grain boundaries, linedefects, vacancies, and other lattice defects, such as those many carbonatoms configured in pentagons, heptagons, or octagons, as opposed to thenormal hexagon. These defects impede the flow of electrons and phonons.For these reasons, the CVD graphene is not considered as pristinegraphene in the scientific community.

Pristine graphene can be produced by direct ultrasonication or liquidphase production, supercritical fluid exfoliation, direct solventdissolution, alkali metal intercalation and water-induced explosion ofnatural graphite particles, or more expensive epitaxial growth. Pristinegraphene is normally single-grain or single-crystalline, having no grainboundaries. Further, pristine graphene essentially does not containoxygen or hydrogen. However, if so desired, the pristine graphene can beoptionally doped with a chemical species, such as boron or nitrogen, tomodify its electronic and optical behavior in a controlled manner.

A hybrid material containing both graphene oxide and CNT was formed intoa thin film by Tung et al [Ref. 20], but the film does not exhibit asatisfactory balance of optical transparency and electricalconductivity. The highest performance film shows optical transmittanceof 92%, but this is achieved at an unacceptable sheet resistance of636Ω/□. The film with the lowest sheet resistance (240Ω/□ with un-dopedRGO) shows 60% optical transmittance, which is not useful at all. Thegraphene component was prepared from heavily oxidized graphite, whichwas then intensely reduced with hydrazine.

Another hybrid material, containing non-pristine graphene (obtained byCVD) and silver nanowires, was formed into a film [Ref. 22]. Again,CVD-grown graphene is a polycrystalline material (not single-crystallineand not pristine) with many topological defects, such as non-hexagonalcarbon atoms, vacancies, dislocations, and grain boundaries. Grainboundaries in graphene are line defects at the interfaces between twodomains with different crystallographic orientations. Due to theprocessing conditions inherent to the CVD process, the CVD graphene alsocontains non-carbon elements (e.g. hydrogen) and non-hexagonal carbonatoms. All these characteristics (defects and impurities) cansignificantly impede the transport of electrons and phonons in CVDgraphene films. Even with the help from silver nanowires, the best CVDgraphene-AgNW hybrid film exhibits a sheet resistance value that isstill far away from what can be theoretically achieved with graphenealone [Ref. 22]. Besides, CVD processes are slow and expensive.

As discussed above, the CNT mesh, metal nanowire mesh, CVD graphenefilm, GO film (including RGO film), CNT-graphite flake mesh,CNT-graphene oxide (GO) hybrid, and RGO-protected Ag nanowire mesh havebeen proposed to serve as a transparent and conductive electrode, butnone has met the stringent combined requirements of transparency,conductivity, oxidation resistance or long-term stability, mechanicalintegrity and flexibility, surface quality, chemical purity, processease, and low cost.

Thus, it is an object of the present invention to provide a grapheneoxide gel-derived hybrid film containing metal nanowires that meets mostor all of the aforementioned requirements. The graphene oxide gelunexpectedly acts to remove the otherwise readily formed metal oxide orother metal compounds that are highly insulating and lead to anextremely high contact resistance.

It is another object of the present invention to provide a process forproducing a graphene oxide gel-derived hybrid film (containing grapheneoxide or reduced graphene oxide) that is a variable alternative to ITO.Surprisingly, this process inherently removes metal oxide or metalcompounds that automatically form on the surface of metal nanowires(e.g. Ag and Cu nanowires) when exposed to open air. This process alsoenables the coverage of metal nanowires (after removal of metal oxide orother metal compounds) with the RGO film, preventing the surface ofmetal nanowires from re-exposing to open air or any other oxidizingenvironment.

SUMMARY OF THE INVENTION

The present invention provides a process for producing an opticallytransparent and electrically conductive film. The process comprises: (a)providing a graphene oxide gel prepared from oxidation of a graphiticmaterial in an oxidizing medium wherein the graphene oxide gel containsgraphene oxide molecules dissolved in a liquid medium; (b) dispersingmetal nanowires in the graphene oxide gel to form a suspension; (c)dispensing and depositing the suspension onto a supporting substrate;and (d) removing the liquid medium from the suspension to form thedesired optically transparent and electrically conductive film. Thisfilm is composed of metal nanowires and graphene oxide (GO or RGO) witha metal nanowire-to-graphene oxide weight ratio of from 1/99 to 99/1,wherein the metal nanowires contain no surface-borne metal oxide ormetal compound and the film exhibits an optical transparence no lessthan 80% and sheet resistance no higher than 300 ohm/square.

Graphene oxide gel can be produced by dispersing graphite particles inan oxidizing liquid (e.g. mixture of strong sulfuric acid, nitric acid,and potassium permanganate) for a sufficient length of time. Theoriginal graphite particles can be heavily oxidized, to the extent thatpractically every one of the original graphene planes has been oxidizedand isolated from one another to become individual molecules thatpossess highly reactive functional groups at the edge and, mostly, ongraphene planes as well. These individual hydrocarbon molecules(containing elements such as O and H, not just carbon atoms) aredissolved in the reaction medium (e.g. mixture of water and acids) toform a gel-like mass, herein referred to as GO gel.

We have most surprisingly observed that GO gel appears to be a goodreducing agent or cleaning agent, capable of removing metal oxide orother metal compound from metal nanowire surfaces. These metal oxide ormetal compounds are electrically insulating, preventing intimatemetal-to-metal contacts when these nanowires are deposited to form wiremeshes and also preventing metal-to-graphene contacts when thesenanowires are co-deposited with a graphene material using a conventionalprocess. Instead, by immersing metal nanowires in GO gel, these metaloxides or compounds are removed, enabling intimate metal-to-metalcontacts and metal-to-graphene contacts when the metal nanowire-grapheneoxide gel suspension is deposited and dried.

In general, the graphene oxide gel is composed of graphene oxidemolecules dispersed in an acidic medium having a pH value of no higherthan 5 and the graphene oxide molecules have an oxygen content no lessthan 20% by weight while in a gel state.

The GO gel is obtained by immersing a graphitic material in a powder orfibrous form (e.g. natural or artificial graphite powder or graphitefibers) in an oxidizing liquid medium in a reaction vessel at a reactiontemperature for a length of time sufficient to obtain a graphene oxidegel composed of graphene oxide molecules dispersed in the liquid medium.The graphene oxide molecules preferably and typically have an oxygencontent no less than 20% by weight (typically 20%-46% by weight ofoxygen) and a molecular weight less than 43,000 g/mole while in a gelstate. Preferably, graphene oxide molecules have a molecular weight lessthan 4,000 g/mole while in a gel state, more preferably between 200g/mole and 4,000 g/mole while in a gel state.

The starting materials for the preparation of graphene oxide gel caninclude a graphitic material selected from natural graphite, artificialgraphite, meso-phase carbon, meso-phase pitch, meso-carbon micro-bead,soft carbon, hard carbon, coke, carbon fiber, carbon nano-fiber, carbonnano-tube, or a combination thereof. Typically, when graphite particlesare immersed in an oxidizing agent, the resulting suspension isinitially optically opaque. As the oxidizing reaction proceeds to acritical extent, an optically transparent or translucent solution isformed. This graphene oxide gel has the characteristics that it isvisually homogeneous with no discernible discrete graphene or grapheneoxide sheets dispersed therein. In contrast, conventional suspension ofdiscrete graphene or graphene oxide sheets, or graphite flakes dispersedin a liquid like water looks opaque, dark, black or heavy brown in colorwith individual graphene sheets, graphene oxide sheets, or graphiteflakes being discernible or recognizable with naked eyes.

The graphene oxide molecules dissolved in the liquid medium of agraphene oxide gel are aromatic chains that have an average number ofbenzene rings in the chain typically less than 1000, more typically lessthan 500, and most typically less than 100. Most of the molecules havemore than 5 or 6 benzene rings (mostly >10 benzene rings) from combinedatomic force microscopy, high-resolution TEM, and molecular weightmeasurements. These benzene-ring type of aromatic molecules have beenheavily oxidized and contain functional groups, such as —COOH and —OHand, therefore, are “soluble” (not just dispersible) in polar solvents,such as water.

These soluble molecules behave like resins and are surprisingly capableof forming a coherent layer of graphene oxide of good structuralintegrity and high thermal conductivity. By contrast, conventionaldiscrete graphene or graphene oxide sheets (including RGO sheets) insolid powder form and graphite flakes do not have any self-adhesion orcohesion power. These sheets or flakes are not living molecules and arerelatively inactive or chemically inert.

The graphene oxide (including RGO) produced herein via GO gel route istypically single-layer graphene but can contain some few-layer graphene(up to 10 planes of carbon atoms, but more typically up to 5 planes). Itis important to point out that the number of graphene planes of atoms(i.e. the number of hexagonal planes of carbon atoms) in a grapheneplatelet is an essential factor that differentiates one type of graphenefrom another type. A single-layer graphene is a semi-metal with a zeroenergy gap. A double-layer graphene sheet/platelet is a semiconductorwith a non-zero energy gap. As the number of graphene planes increasesfrom 3 to 10 layers, the graphene material is transitioned from asemiconductor to a metal (or conductor). Hence, single-layer graphene,double-layer graphene, few-layer graphene (3-10 layers), and thickergraphene platelets (>10 layers) are all considered as distinct anddifferent types of materials in scientific community.

Preferably and typically, a thin film (preferably <5 nm and morepreferably <2 nm) containing GO gel-derived RGO and metal nanowiresexhibits an optical transparence no less than 85% and sheet resistanceno higher than 300 ohm/square. More preferably and more typically, thefilm exhibits an optical transparence no less than 85% and sheetresistance no higher than 200 ohm/square (preferably no higher than 100ohm/square). Still more preferably and typically, the film exhibits anoptical transparence no less than 90% and sheet resistance no higherthan 200 ohm/square (preferably no higher than 100 ohm/square andfurther preferably no higher than 50 ohm/square). In many cases, thefilm exhibits an optical transparence no less than 95% at a sheetresistance no higher than 100 ohm/square (often no higher than 50ohm/square).

Most surprisingly, some of our best films derived from GO gel achieve asheet resistance of 63 ohm/square at 95% transparence and 74 ohm/squareat 97% transparency. The lowest sheet resistance achieved withGO-derived films thus far has been 19 ohm/square. These performancelevels are outstanding and have exceeded the very best values of allgraphene oxide-based hybrid material films ever reported. These valuesare comparable to those of ITO-based films, yet the presently inventedgraphene-based hybrid films are flexible, durable, mechanical strong,environmentally stable, and potentially much less expensive.

For the purpose of defining the claim languages, the “metal nanowire” inthe instant application refers to an elongated metal nanostructurehaving a largest dimension (e.g. length) and a smallest dimension (e.g.thickness or diameter) wherein the largest dimension-to-smallestdimension ratio is no less than 3 and the smallest dimension is nogreater than 200 nm (preferably no greater than 100 nm). The metalnanowire refers to a metal nanostructure having a rod, wire, sheet,belt, or tube shape that meets the aforementioned dimensionrequirements. In other words, the metal nanowire can include nano-rod,nano-wire, nano-sheet, nano-belt, or nano-tube.

In a preferred embodiment, the metal nanowires are selected fromnanowires of silver (Ag), gold (Au), copper (Cu), platinum (Pt), zinc(Zn), cadmium (Cd), cobalt (Co), molybdenum (Mo), aluminum (Al), analloy thereof, or a combination thereof. Particularly desired metalnanowires are silver nanowires, but metal nanowires may be selected fromnanowires of a transition metal or an alloy of a transition metal.

The optically transparent and electrically conductive film may be afree-standing film or supported on a transparent substrate. In apreferred configuration, the film is supported on a transparentsubstrate and the mesh or meshes of metal nanowires are embedded in theRGO derived from GO gel.

The hybrid metal nanowire/RGO-based optically transparent andelectrically conductive film may further contain carbon nanotubes orcarbon nano-fibers.

The present invention also provides an optically transparent andelectrically conductive film produced by the process described above. Inone embodiment, this film is composed of metal nanowires and grapheneoxide (including reduced graphene oxide) having a metalnanowire-to-graphene oxide weight ratio of from 1/9 to 9/1, wherein themetal nanowires contain no surface-borne metal oxide or metal compoundand said film exhibits an optical transparence no less than 80% andsheet resistance no higher than 200 ohm/square (preferably no higherthan 100 ohm/square). The film may further contain carbon nanotubes orcarbon nano-fibers.

The invention further provides an electro-optic device containing anoptically transparent and electrically conductive film herein producedas an electrode. The electro-optic device may be a photovoltaic or solarcell, light-emitting diode, photo-detector, touch screen,electro-wetting display, liquid crystal display, plasma display, LEDdisplay, a TV screen, a computer screen, or a mobile phone screen.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(a) A flow chart illustrating various prior art processes forproducing nano graphene platelets (graphene oxide, reduced grapheneoxide, and pristine graphene) and exfoliated graphite products (flexiblegraphite foils and flexible graphite composites); (b) Schematic drawingillustrating the processes for producing a film or membrane of simplyaggregated graphite or NGP flakes/platelets; all processes begin withintercalation and/or oxidation treatment of graphitic materials (e.g.natural graphite particles).

FIG. 2(a) Sheet resistance and optical transmittance (at 550 nmwavelength) of various AgNW, conventional AgNW-RGO, and the new AgNW-RGO(GO gel derived) films; (b) Sheet resistance and optical transmittance(at 550 nm wavelength) of various conventional AgNW-RGO and new AgNW-RGO(GO gel derived) films.

FIG. 3 Sheet resistance and optical transmittance (at 550 nm wavelength)of various CuNW, CuNW-RGO (conventional), and new CuNW-RGO (GO gelderived) films.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A preferred embodiment of the present invention is a process forproducing an optically transparent and electrically conductive filmcomposed of a mixture or hybrid of metal nanowires and graphene oxide(GO) or reduced graphene oxide (RGO) derived from GO gel. The metalnanowire-to-graphene oxide (or RGO) weight ratio in this mixture is from1/99 to 99/1. The film exhibits an optical transparence no less than 80%and sheet resistance no higher than 300 ohm/square. The film istypically thinner than 1 μm, more often thinner than 100 nm, even moreoften and preferably thinner than 10 nm, most often thinner than 1 nm,and can be as thin as 0.34 nm. The metal nanowires contain nosurface-borne metal oxide or metal compound that otherwise areelectrically insulating.

As a preferred embodiment, the process comprises: (a) providing agraphene oxide gel prepared from oxidation of a graphitic material in anoxidizing medium wherein the graphene oxide gel contains graphene oxidemolecules dissolved in a liquid medium; (b) dispersing metal nanowiresin the graphene oxide gel to form a suspension; (c) dispensing anddepositing the suspension onto a supporting substrate; and (d) removingthe liquid medium from the suspension to form the desired opticallytransparent and electrically conductive film.

The two key components in this transparent and conductive film are metalnanowires (e.g. silver nanowires, AgNW) and GO (or RGO). This GO or RGOis derived from GO gel, which will be described first.

Graphene normally refers to a sheet of carbon atoms that are arranged ina hexagonal lattice and the sheet is one carbon atom thick. Thisisolated, individual plane of carbon atoms is commonly referred to assingle-layer graphene. A stack of multiple graphene planes bondedthrough van der Waals forces in the thickness direction with aninter-graphene plane spacing of 0.3354 nm is commonly referred to as amulti-layer graphene. A multi-layer graphene platelet has up to 300layers of graphene planes (<100 nm in thickness). When the platelet hasup to 5-10 graphene planes, it is commonly referred to as “few-layergraphene” in the scientific community. Single-layer graphene andmulti-layer graphene sheets are collectively called “nano grapheneplatelets” (NGPs). Graphene sheets/platelets or NGPs are a new class ofcarbon nano material (a 2-D nano carbon) that is distinct from the 0-Dfullerene, the 1-D CNT, and the 3-D graphite.

In the instant application and in keeping with commonly accepteddefinitions in scientific community, NGPs or graphene materials caninclude discrete sheets/platelets of single-layer and multi-layerpristine graphene, graphene oxide, or reduced graphene oxide withdifferent oxygen contents. Pristine graphene has essentially 0% oxygenand 0% hydrogen. Graphene oxide (GO) has 0.01%-46% by weight of oxygenand reduced graphene oxide (RGO) has 0.01%-2.0% by weight of oxygen. Inother words, RGO is a type of GO having lower but non-zero oxygencontent. Additionally, both GO and RGO contain a high population ofedge- and surface-borne chemical groups, vacancies, oxidative traps, andother types of defects, and both GO and RGO contain oxygen and othernon-carbon elements, e.g. hydrogen. In contrast, the pristine graphenesheets are practically defect-free on the graphene plane and contain nooxygen. Hence, GO and RGO are commonly considered in the scientificcommunity as a class of 2-D nano material that is fundamentallydifferent and distinct from pristine graphene.

NGPs are commonly obtained by intercalating natural graphite particleswith a strong acid and/or oxidizing agent to obtain a graphiteintercalation compound (GIC) or graphite oxide (GO), as illustrated inFIG. 1(a) (process flow chart) and FIG. 1(b) (schematic drawing). Thepresence of chemical species or functional groups in the interstitialspaces between graphene planes serves to increase the inter-graphenespacing (d₀₀₂, as determined by X-ray diffraction), therebysignificantly reducing the van der Waals forces that otherwise holdgraphene planes together along the crystallographic c-axis direction.The GIC or GO is most often produced by immersing natural graphitepowder (20 in FIGS. 1(a) and 100 in FIG. 1(b)) in a mixture of sulfuricacid, nitric acid (an oxidizing agent), and another oxidizing agent(e.g. potassium permanganate or sodium perchlorate). The resulting GIC(22 or 102) is actually some type of graphite oxide (GO) particles.Strong oxidation of graphite particles can result in the formation of agel-like state called “GO gel” 21. The GIC 22 is then repeatedly washedand rinsed in water to remove excess acids, resulting in a graphiteoxide suspension or dispersion, which contains discrete and visuallydiscernible graphite oxide particles dispersed in water. There are twoprocessing routes to follow after this rinsing step:

Route 1 involves removing water from the graphite oxide suspension toobtain “expandable graphite,” which is essentially a mass of dried GICor dried graphite oxide particles. Upon exposure of expandable graphiteto a temperature in the range of typically 800-1,050° C. forapproximately 30 seconds to 2 minutes, the GIC undergoes a rapidexpansion by a factor of 30-300 to form “graphite worms” (24 or 104),which are each a collection of exfoliated, but largely un-separatedgraphite flakes that remain interconnected.

In Route 1A, these graphite worms (exfoliated graphite or “networks ofinterconnected/non-separated graphite flakes”) can be re-compressed toobtain flexible graphite sheets or foils (26 or 106) that typically havea thickness in the range of 0.1 mm (100 μm)-0.5 mm (500 μm).Alternatively, one may choose to use a low-intensity air mill orshearing machine to simply break up the graphite worms for the purposeof producing the so-called “expanded graphite flakes” (49 or 108) whichcontain mostly graphite flakes or platelets thicker than 100 nm (hence,not a nano material by definition).

Exfoliated graphite worms, expanded graphite flakes, and therecompressed mass of graphite worms (commonly referred to as flexiblegraphite sheet or flexible graphite foil) are all 3-D graphiticmaterials that are fundamentally different and patently distinct fromeither the 1-D nano carbon material (CNT or CNF) or the 2-D nano carbonmaterial (graphene sheets or platelets, NGPs). Flexible graphite (FG)foils are completely opaque and cannot be used as a transparentelectrode.

In Route 1B, the exfoliated graphite is subjected to high-intensitymechanical shearing (e.g. using an ultrasonicator, high-shear mixer,high-intensity air jet mill, or high-energy ball mill) to form separatedsingle-layer and multi-layer graphene sheets (collectively called NGPs,33 or 112), as disclosed in our U.S. application Ser. No. 10/858,814.Single-layer graphene can be as thin as 0.34 nm, while multi-layergraphene can have a thickness up to 100 nm. In the present application,the thickness of multi-layer NGPs is typically less than 20 nm. The NGPs(still containing oxygen) may be dispersed in a liquid medium and castinto a GO thin film 34.

Route 2 entails ultrasonicating the graphite oxide suspension for thepurpose of separating/isolating individual graphene oxide sheets fromgraphite oxide particles. This is based on the notion that theinter-graphene plane separation has been increased from 0.3354 nm innatural graphite to 0.6-1.1 nm in highly oxidized graphite oxide,significantly weakening the van der Waals forces that hold neighboringplanes together. Ultrasonic power can be sufficient to further separategraphene plane sheets to form separated, isolated, or discrete grapheneoxide (GO) sheets. These graphene oxide sheets can then be chemically orthermally reduced to obtain “reduced graphene oxides” (RGO) typicallyhaving an oxygen content of 0.01%-10% by weight, more typically 0.01%-5%by weight, and most typically 0.01%-2.0% by weight of oxygen with heavychemical reduction using a reducing agent like hydrazine. In thescientific community, the chemically processed graphene-basedtransparent and conductive electrode normally refers to the RGO producedin this manner (as opposed to CVD deposited).

It is important to further emphasize the fact that, in the typical priorart processes, ultrasonification is used after intercalation andoxidation of graphite (i.e., after first expansion) and most typicallyafter thermal shock exposure of the resulting GIC or GO (i.e., aftersecond expansion or exfoliation) to aid in breaking up those graphiteworms. There are already much larger spacings between flakes afterintercalation and/or after exfoliation (hence, making it possible toeasily separate flakes by ultrasonic waves). This ultrasonication wasnot perceived to be capable of separating thoseun-intercalated/un-oxidized layers where the inter-graphene spacingremains <0.34 nm and the van der Waals forces remain strong.

The applicant's research group was the very first in the world tosurprisingly observe that, under proper conditions (e.g., with anultrasonic frequency and intensity and under the assistance of a certaintype of surfactant), ultrasonication can be used to produce ultra-thingraphene directly from graphite, without having to go through chemicalintercalation or oxidation. This invention was reported in a patentapplication [A. Zhamu, et al., “Method of Producing Exfoliated Graphite,Flexible Graphite, and Nano Graphene Plates,” U.S. patent Ser. No.11/800,728 (May 8, 2007); now U.S. Pat. No. 7,824,651 (Nov. 2, 2010)].This “direct ultrasonication” process is capable of producing bothsingle-layer and few-layer pristine graphene sheets. This innovativeprocess involves simply dispersing pristine graphite powder particles 20in a liquid medium (e.g., water, alcohol, or acetone) containing adispersing agent or surfactant to obtain a suspension. The suspension isthen subjected to an ultrasonication treatment, typically at atemperature between 0° C. and 100° C. for 10-120 minutes, resulting inultra-thin pristine graphene sheets suspended in a liquid medium. Theresulting suspension can be cast to form a pristine graphene film 38. Nochemical intercalation or oxidation is required. The graphite materialhas never been exposed to any obnoxious chemical. This process combinesexpansion, exfoliation, and separation into one step. Hence, this simpleyet elegant method obviates the need to expose graphite to ahigh-temperature, or chemical oxidizing environment. Upon drying, theresulting NGPs are essentially pristine graphene, containing no oxygenand no surface defects. These pristine graphene sheets, single-layer ormulti-layer, are all highly conductive both electrically and thermally.

The graphene oxide gel is a very unique and novel class of material thatsurprisingly has great cohesion power (self-bonding, self-polymerizing,and self-crosslinking capability) and adhesive power (capable ofchemically bonding to a wide variety of solid surfaces). Thesecharacteristics have not been taught or hinted in the prior art. The GOgel is obtained by immersing powders or filaments of a startinggraphitic material in an oxidizing liquid medium (e.g. a mixture ofsulfuric acid, nitric acid, and potassium permanganate) in a reactionvessel. The starting graphitic material may be selected from naturalgraphite, artificial graphite, meso-phase carbon, meso-phase pitch,meso-carbon micro-bead, soft carbon, hard carbon, coke, carbon fiber,carbon nano-fiber, carbon nano-tube, or a combination thereof.

When the starting graphite powders or filaments are mixed in theoxidizing liquid medium, the resulting slurry initially appearscompletely dark and opaque. When the oxidation of graphite proceeds at areaction temperature for a sufficient length of time under a controlledpH condition, the reacting mass can eventually turn opticallytranslucent or transparent, which also looks and behaves like a gel.This heavy oxidation-induced graphene oxide gel is composed of grapheneoxide molecules dispersed in the liquid medium. The graphene oxidemolecules, prior to any subsequent heat treatment, have an oxygencontent no less than 20% by weight (typically from 40-50% by weight) andtheir molecular weights are typically less than 43,000 g/mole (oftenless than 4,000 g/mole, but typically greater than 200 g/mole) while ina gel state. The graphene oxide gel is composed of graphene oxidemolecules dispersed in an acidic medium having a pH value of typicallyno higher than 5.

This gel state can be converted to a solid state via adding a chemicalreducing agent (e.g. hydrazine or sodium boron hydride) to reduce the GOmolecules to RGO sheets, which precipitate out of the liquid medium.Upon removal of the liquid, the resulting product is RGO powder.Alternatively, the GO gel can be just boiled for an extended period oftime (e.g. >1 hour) to precipitate out the partially reduced GO. Byremoving the liquid component one obtains the partially reduced GO,which can be further heat-treated to produce fully reduced RGO. The RGOpowder produced by either approach can be re-dispersed in a solvent withthe assistance of a surfactant or dispersing agent to form a suspension,which can be cast or spin-coated to form RGO films. Initially, thesecommonly accepted procedures were what we used to prepare thin RGO filmsor RGO-protected metal nanowire films. However, the sheet resistance andoptical transparency of the films produced in this manner were notsatisfactory.

We then decided to undertake a drastically different approach. In thisnovel approach, we began with the preparation of GO gel, but we did notthermally or chemically reduce the GO gel prior to mixing metalnanowires into the GO gel. Instead, we mixed metal nanowires into GO gelprior to GO reduction or removal of liquid from GO gel. This strategyallows metal nanowires to interact with the living GO molecules in thegel state. The original intent was to provide an environment that wasconducive to the formation of a strong bond between metal nanowires andthe GO sheets when the GO was subsequently reduced. Indeed, we were ableto accomplish this original goal (of having strong interfacial bondsbetween metal nanowires and GO sheets). However, quite surprisingly,this approach also led to an unexpected result that the metalnanowire-RGO films exhibited an outstanding combination of hightransparency and low sheet resistance, significantly better than thoseof the corresponding metal nanowire-RGO films prepared earlier (i.e.,those films prepared through reduction of GO first and then mixing theresulting RGO with metal nanowires).

After this surprising discovery, we proceeded to conduct some analysisto gain an understanding why the GO gel route was so powerful. Chemicalanalyses have demonstrated that prior to immersion into GO gel, silvernanowires had a thin but finite layer (scale) of silver oxide. Byimmersing these silver nanowires in GO gel for 2 hours or longer, thesilver oxide scale disappeared. Untreated copper nanowires also showedsome copper oxide and copper chloride compound, which were removed uponimmersion in GO gel at a temperature higher than 23° C. for 0.5-2 hours.Removal of the insulating scale from metal nanowire surfaces enablesintimate metal-to-graphene and metal-to-metal contacts with minimalcontact resistance. This is likely a major reason for the observedperformance of GO gel-derived thin films.

There are many processes, with or without a template, that can be usedto produce metal nanowires, and these are well known in the art. Awidely used approach to fabricate metal nanowires is based on the use ofvarious templates, which include negative, positive, and surface steptemplates. Negative template methods use prefabricated cylindricalnano-pores in a solid material as templates. By depositing metals intothe nano-pores, nanowires with a diameter predetermined by the diameterof the nano-pores are fabricated.

The positive template method uses wire-like nanostructures, such as DNAand carbon nanotubes as templates, and nanowires are formed on the outersurface of the templates. Unlike negative templates, the diameters ofthe nanowires are not restricted by the template sizes and can becontrolled by adjusting the amount of materials deposited on thetemplates. By removing the templates after deposition, wire-like andtube-like structures can be formed.

Atomic-scale step edges on a crystal surface can be used as templates togrow nanowires. The method takes advantage of the fact that depositionof many materials on a surface often starts preferentially at defectsites, such as surface step-edges. For this reason, the method issometimes called “step edge decoration.” As examples, several researchgroups prepared metal nanowires on vicinal single crystal surfaces usingthe physical vapor deposition (PVD) method. Others fabricated metalnanowires of 1-2 atomic layer thick with a controlled “width” and wirespacing.

Metal nanowires (optionally containing other conductive additives, suchas CNTs and carbon nano-fibers) may be dispersed in a GO gel. Theresulting suspension is then dispensed and deposited onto a solidsubstrate. As a control sample, GO or RGO platelets may be dispersed ina liquid medium (e.g. water) with or without a surfactant to form asuspension or ink. Metal nanowires are then mixed into this non-GO gelsuspension, which is then dispensed and deposited onto a supportingsubstrate. As another control samples, metal nanowire suspension isdeposited onto a supporting substrate to form a thin film or mesh ofnanowires. This nanowire thin film is then covered with a subsequentlydeposited RGO film (not from the GO gel route). The results are quitestriking, as further discussed in the examples presented toward the endof the section.

The surface of many metal nanowires can be effectively cleaned by usingGO gel. Examples include silver (Ag), gold (Au), copper (Cu), platinum(Pt), zinc (Zn), cadmium (Cd), cobalt (Co), molybdenum (Mo), aluminum(Al), and their alloys.

The RGO-, metal nanowire-, RGO-metal nanowire, and GO-derived RGO-metalnanowire films may be deposited from the suspension or ink using avariety of methods, including spray painting, drop casting, spincoating, vacuum-assisted filtration, electrostatic deposition, and dipcoating.

In a spray painting process, the solution/suspension/ink can be spraypainted onto a heated or non-heated substrate. The substrate may berinsed during the spraying process to remove the solubilization agent,or surfactant. The spraying solution/suspension/ink may be of anyconcentration. The substrate surface may be functionalized to aid inadhesion of the deposited species (metal nanowires, CNTs, and/or GO).The spraying rate and the number of spraying passes may be varied toobtain different amounts of deposited species.

In a drop casting process, a drop of the solution/suspension/ink can beplaced onto a substrate for a period of time. The substrate may befunctionalized to enhance adhesion of deposited species. The substratewith graphene may be rinsed by appropriate solvents. Alternatively, thesuspension can be spin-coated along with an appropriate solvent toremove the surfactant simultaneously. In dip coating, the supportingsubstrate can be dipped into the suspension for a period of time. Thismay form a film of RGO or RGO/nanowire hybrids. In a printing process,the film may be transferred from one substrate to another by means of astamp. The stamp may be made from Polydimethyl-siloxane (PDMS). Thetransfer can be aided by gentle heating (up to 100° C.) and pressure. Ina vacuum filtration process, the suspension/ink can be filtered througha porous membrane under the assistance of a vacuum pump. A film of RGO,or RGO-nanowire hybrid is deposited on top of the filtering membrane.The film can be washed while on the filter with a liquid medium toremove surfactant, functionalization agents, or unwanted impurities.

The following examples serve to provide the best modes of practice forthe present invention and should not be construed as limiting the scopeof the invention:

EXAMPLE 1 Thermal Exfoliation and Separation of Graphite Oxide

Graphite oxide was prepared by oxidation of graphite flakes withsulfuric acid, nitrate, and permanganate according to the method ofHummers [U.S. Pat. No. 2,798,878, Jul. 9, 1957]. Upon completion of thereaction, the mixture was poured into deionized water and filtered. Thegraphite oxide was repeatedly washed in a 5% solution of HCl to removemost of the sulphate ions. The GO sample was then washed repeatedly withdeionized water until the pH of the filtrate was neutral. After thisprocedure, we followed two routes to prepare RGO and RGO-metal nanowiresuspensions:

In the first route, portion of the GO-deionized water suspension (not aGO gel) was cast onto a glass surface to form a GO film, which was thenthermally reduced at 150° C. for 2 hours in vacuum and then 500° C. in aflowing nitrogen/hydrogen condition in a tube furnace to obtain a RGOfilm. Metal nanowires were poured into the remaining portion of the GOsuspension, which was cast to form GO-metal nanowire films and thensubjected to identical thermal reduction treatments.

In the second route, portion of the GO-deionized water suspension wasspray-dried and stored in a vacuum oven at 60° C. for 24 hours. Theinterlayer spacing of the resulting laminar graphite oxide wasdetermined by the Debye-Scherrer X-ray technique to be approximately0.73 nm (7.3 Å). Dried graphite oxide powder was then placed in a tubefurnace pre-set at a temperature of 1,050° C. for 60 minutes. Theresulting exfoliated graphite was subjected to low-power ultrasonication(60 watts) for 10 minutes to break up the graphite worms and separategraphene oxide layers. These GO sheets actually are partially reduced GOsince the high exfoliation temperatures allowed some oxygen-containingfunctional groups to get released. Several batches of graphite oxide(GO) platelets were produced under identical conditions to obtainapproximately 2.4 Kg of oxidized NGPs (GO) platelets. Portions of theseGO platelets, with and without metal nanowires, were re-dispersed inwater in the presence of a surfactant and cast to form RGO and RGO-metalnanowire films.

EXAMPLE 2 Preparation of Graphene Oxide (GO) Gel

Graphite oxide gel was prepared by oxidation of graphite flakes with anoxidizer liquid consisting of sulfuric acid, sodium nitrate, andpotassium permanganate at a ratio of 4:1:0.05 at 30° C. When naturalgraphite flakes (particle sizes of <14 μm) were immersed and dispersedin the oxidizer mixture liquid, the suspension or slurry initiallyappeared optically opaque and dark. The suspension remained opaqueduring the first 52 hours of reaction. However, the suspension graduallyturned optically translucent (a little cloudy) when the reaction timeexceeded 52 hours, and the color of the suspension changed from black todark brown. After 96 hours, the suspension suddenly became an opticallytransparent solution with light brown color. The solution appeared veryuniform in color and transparency, indicating the absence of anydispersed discrete objects. The whole solution behaves like a gel, verysimilar to a typical polymer gel.

Surprisingly, by casting this GO gel or the GO gel containing metalnanowires on a glass surface and removing the liquid medium from thecast film we obtain a thin film of graphene oxide or GO-metal nanowirethat is optically transparent. These thin films look like, feel like,and behave like regular polymer thin films. These films were thenthermally reduced at 150° C. for 2 hours in vacuum and then 500° C. in aflowing nitrogen/hydrogen condition in a tube furnace to obtainGO-derived RGO or GO-derived RGO-metal nanowire films.

EXAMPLE 3 Characterization of Thin Films of Silver Nanowires (AgNW),AgNW/RGO Hybrid (Conventional), and AgNW/RGO (GO-Derived) HybridMaterials

Silver nanowires were purchased from Seashell Technologies (La Jolla,Calif., USA) as suspension in isopropyl alcohol with concentrations of25 mg/ml. A small volume of dispersion was diluted down to approximately1 mg/ml with isopropyl alcohol. This was subjected to half-an-hoursonication in a sonic bath. Then, this suspension was applied to a 50mm×100 mm poly(ethylene terephthalate) (PET) substrates by a manuallycontrolled wire-wound, i.e., pushing the suspension on top of thesubstrate with a rod.

In addition, AgNW films were prepared by spin-coating AgNW inks on glasssubstrates. To prepare AgNW films on glass substrates, we treated glasssubstrates with UV/Ozone to make hydrophilic surfaces for AgNW. Then,AgNW ink was spin-coated on a glass substrate and dried at 120° C. for 5min. Several AgNW films were prepared by changing spin-coating speedfrom 250 to 2,000 rpm to investigate the effect of spin-coating speed onoptical and electrical properties of AgNW films. Transparent electrodefilms of AgNW-RGO and AgNW-RGO (GO gel-derived) hybrids were alsoprepared in a similar manner. Separately, the AgNW-RGO hybridtransparent electrode films were prepared by coating RGO or GO gel ontothe AgNW film.

An UV/Vis/NIR was used to measure the optical transmittance of AgNW,AgNW-RGO, and AgNW-(GO gel-derived) films. The sheet resistances weremeasured by a non-contact Rs measurement instrument. The sheetresistance and optical transparency data are summarized in FIGS. 2(a)and 2(b). Several significant observations can be made from thesefigures: (A) The AgNW-(GO gel-derived) films significantly out-performboth AgNW and conventional AgNW-RGO films in terms of high transmittanceand/or low sheet resistance. (B) With hybrid AgNW-(GO gel-derived)films, we were able to achieve a sheet resistance value of 74 and 63Ω/□at 97% and 95% transmittance, respectively. These values are evensuperior to those of un-doped CVD graphene or CVD graphene-AgNW films.These outstanding combined performances are achieved by using highlyscalable, more cost-effective, less tedious, and vacuum equipment-freeprocesses. This is most surprising considering the notion that RGO wouldbe significantly less conducting as compared to CVD graphene. (C) Sheetresistance values as low as 19Ω/□ have been obtained, which arecomparable to those of high-end ITO glass. Yet, thin films of grapheneoxide-enhanced metal nanowires are highly flexible and ductile, incontrast to the rigid and brittle ITO-coated polymer or glass. Thesesurprisingly low sheet resistance values were achieved at an opticaltransmittance higher than 80%.

EXAMPLE 4 Copper Nanowire (CuNW) Film, RGO Film, CuNW/RGO Hybrid Film,CuNW/(GO Gel-Derived) Film

The preparation of CuNW followed the approach of a self-catalytic growthof Cu nanowires within a liquid-crystalline medium of hexadecylamine(HAD) and cetyltriamoninum bromide (CTAB). HDA and CTAB were first mixedat elevated temperature to form a liquid-crystalline medium. Uponaddition of the precursor, copper acetylacetonate [Cu(acac)2], longnanowires with excellent dispersibility form spontaneously within themedium in the presence of a catalytic Pt surface. Specifically, asolution process was followed to prepare copper nanowires (CuNWs). As anexample, 8 g HAD and 0.5 g CTAB were dissolved in a glass vial at 180°C. Then, 200 mg copper acetylacetonate was added and magneticallystirred for 10 minutes. Subsequently, a silicon wafer (0.5 cm2)sputtered with ˜10 nm of platinum was placed into the vial. The mixtureswere then kept at 180° C. for 10 hours, resulting in the formation ofreddish cotton-like sheets settled at the bottom. After rinsing withtoluene for several times, the nanowires were dispersed in toluene atdifferent solid contents. The suspensions were separately cast into thinfilms on glass or PET surface. Several CuNW films supported on glass orPET substrate were then deposited with either RGO film or GO gel-derivedRGO film. Finally, CuNWs were also dispersed into GO gel and cast intothin films and heat treated according to a procedure described inExample 2.

The sheet resistance and optical transparency data are summarized inFIG. 3. Several significant observations can be made by examining thedata from this chart: (A) The CuNW-(GO gel-derived) films significantlyout-perform both CuNW and conventional CuNW-RGO films in terms of hightransmittance and/or low sheet resistance. (B) With hybrid CuNW-RGO (GOgel-derived) films, we were able to achieve sheet resistance values of153 and 124Ω/□ at 93% and 91% transmittance, respectively. These valuesare superior to those of all CuNW-based electrodes ever reported. Theseoutstanding combined performances are achieved by using highly scalable,more cost-effective, less tedious, and vacuum equipment-free processes.(C) Sheet resistance values as low as 44 and 24Ω/□ have been obtained,which are comparable to those of ITO glass. These surprisingly low sheetresistance values were achieved at an optical transmittance of 82% and84%, respectively. These are most impressive and surprising consideringthe fact that the electrical conductivity of Cu is an order of magnitudelower than that of silver and, hence, one would not have expected such alow sheet resistance associated with CuNW even when in combination withgraphene-type material, which is even lower than Cu in electricalconductivity.

In summary, a novel and unique class of transparent and conductiveelectrodes has been developed. This new class of hybrid materialssurprisingly offers the following special features and advantages:

-   (a) Thin films containing networks of metal NWs combined with GO    gel-derived reduced graphene oxide sheets (RGO) prepared through    solution processing techniques are a promising replacement to ITO    glass due to its exceptionally high conductivity (low resistance)    and optical transmittance. The superior performance of GO-derived    RGO-AgNW electrodes with minimal contact resistance (at RGO-AgNW and    AgNW-AgNW interfaces) make them particularly suitable for use in    various electro-optic devices, such as a touch screen for a smart    phone or tablet computer, organic solar cell, photo-detector, and    display device.-   (b) GO gel enables the elimination of electrically insulating metal    oxide or metal compound from metal nanowire surfaces. This    unexpected benefit makes the GO-derived RGO (with metal nanowires    being mixed in GO gel prior to GO reduction) significantly more    effective than many graphene-based materials in terms of imparting    electrical conductance to the metal nanowire films without    compromising the optical transmittance. This has been most    surprising.-   (c) Even though Cu has a much lower electrical conductivity as    compared with silver, the CuNW-RGO electrodes, when prepared from    the GO gel route as herein disclosed, still surprisingly provide    excellent combination of high optical transparency and low sheet    resistance.-   (d) The presently invented RGO-metal NW films are particularly    useful for organic optoelectronic devices such as organic    photovoltaic cells, organic light-emitting diodes, and organic    photo-detectors because they can be deposited on flexible,    light-weight substrates using low-cost fabrication methods. These    films are of high flexibility and structural integrity. We have    observed that these films can be repeatedly bent back and forth for    many times without losing the transmittance and sheet resistance    properties.-   (e) An important aspect of optoelectronic thin-film devices is the    transparent, conductive electrode through which light couples in or    out of the devices. Indium tin oxide (ITO) is widely used but is too    expensive for applications such as solar cells. Moreover, metal    oxides such as ITO are brittle and therefore of limited use on    flexible substrates. The present invention provides a substitute for    ITO since the GO gel-derived RGO-metal NW films deliver similar    sheet resistance and transparency performance, but are of higher    flexibility and lower cost.

We claim:
 1. A process for producing an optically transparent andelectrically conductive film, said process consisting essentially of (a)providing a graphene oxide gel prepared from oxidation of a graphiticmaterial in an oxidizing medium wherein said graphene oxide gel containsgraphene oxide molecules dissolved in a liquid medium; (b) dispersingmetal nanowires in said graphene oxide gel to form a suspension; (c)dispensing and depositing said suspension onto a supporting substrate;and (d) removing said liquid medium from said suspension to form saidoptically transparent and electrically conductive film, which consistsof metal nanowires and graphene oxide having a metalnanowire-to-graphene oxide weight ratio of from 1/99 to 99/1, whereinsaid metal nanowires contain no surface-borne metal oxide or metalcompound and said film exhibits an optical transparence no less than 80%and sheet resistance no higher than 300 ohm/square.
 2. The process ofclaim 1, further comprising a step of thermally and/or chemicallyreducing said graphene oxide to a reduced graphene oxide having anoxygen content no greater than 2% by weight.
 3. The process of claim 1,wherein said metal nanowires are selected from nanowires of silver (Ag),gold (Au), copper (Cu), platinum (Pt), zinc (Zn), cadmium (Cd), cobalt(Co), molybdenum (Mo), aluminum (Al), an alloy thereof, or a combinationthereof.
 4. The process of claim 1, wherein said metal nanowires containsilver nanowires.
 5. The process of claim 1, wherein said metalnanowires contain copper nanowires.
 6. The process of claim 1, whereinsaid metal nanowires are selected from nanowires of a transition metalor an alloy of a transition metal.
 7. The process of claim 1, whereinsaid graphene oxide contains single-layer graphene or few-layer graphenehaving less than 10 graphene planes of atoms.
 8. The process of claim 1,wherein said graphene oxide gel is composed of graphene oxide moleculesdispersed in an acidic medium having a pH value of no higher than 5 andsaid graphene oxide molecules have an oxygen content no less than 20% byweight while in a gel state.
 9. The process of claim 1, wherein saidgraphene oxide gel is obtained by immersing a graphitic material in apowder or fibrous form in an oxidizing liquid medium in a reactionvessel at a reaction temperature for a length of time sufficient toobtain a graphene oxide gel composed of graphene oxide moleculesdispersed in the liquid medium and said graphene oxide molecules have anoxygen content no less than 20% by weight and a molecular weight lessthan 43,000 g/mole while in a gel state.
 10. The process of claim 1,wherein said graphene oxide molecules have a molecular weight less than4,000 g/mole while in a gel state.
 11. The process of claim 1, whereinsaid graphene oxide molecules have a molecular weight between 200 g/moleand 4,000 g/mole while in a gel state.
 12. The process of claim 1,wherein step (c) of dispensing and depositing contains a coating orcasting procedure.
 13. The process of claim 1, wherein step (c) ofdispensing and depositing contains a procedure selected from spraypainting, drop casting, spin coating, vacuum-assisted filtration,electrostatic deposition, or dip coating.
 14. The process of claim 1,wherein said film exhibits an optical transparence no less than 85% andsheet resistance no higher than 100 ohm/square.
 15. The process of claim1, wherein said film exhibits an optical transparence no less than 85%and sheet resistance no higher than 50 ohm/square.
 16. The process ofclaim 1, wherein said film exhibits an optical transparence no less than90% and sheet resistance no higher than 200 ohm/square.
 17. The processof claim 1, wherein said film exhibits an optical transparence no lessthan 90% and sheet resistance no higher than 100 ohm/square.
 18. Theprocess of claim 1, wherein said film exhibits an optical transparenceno less than 95% and sheet resistance no higher than 100 ohm/square. 19.The process of claim 1, wherein said supporting substrate is opticallytransparent.
 20. The process of claim 1, wherein said suspension furthercontains carbon nanotubes or carbon nano-fibers.
 21. The process ofclaim 1, wherein said supporting substrate is moved from a feeder rollerto a collector roller and the process contains a roll-to-roll procedure.